1. Field of the Invention
The present invention relates to an electrophotographic apparatus such as a printing press, copier, printer or facsimile machine and, more particularly, to an electrophotographic apparatus in which after a toner image developed on a peripheral surface of a photoconductor drum is transferred onto a transfer roller, the toner image transferred onto the transfer roller is transferred onto a medium such as a web of paper or a film passing between the transfer roller and a backup roller driven to rotate in contact with the transfer roller.
2. Description of the Related Art
In FIG. 1 there is diagrammatically shown a conventional wet electrophotographic apparatus using a photoconductor drum, a transfer roller and a backup roller. In the Figure, numeral 1 denotes the photoconductor drum, numeral 2 denotes the transfer roller in rotational contact with the photoconductor drum 1, and numeral 3 denotes the backup roller in rotational contact with the transfer roller 2. See JP 2009-157176 A.
In the electrophotographic apparatus in image forming, the photoconductor drum 1 is driven by drive means such as a motor (not shown) to rotate at a fixed speed in the direction of arrow. The photoconductor drum 1 has a peripheral surface charged uniformly by a charging unit 4 in dark and then irradiated with light from an exposure unit 5 to form an original light figure on the peripheral surface of the photoconductor drum 1 to form an electrostatic latent image on the peripheral surface. Thereafter, the electrostatic latent image as it passes through a development region is visualized with a liquid toner by a developing unit 6, forming a toner image on the peripheral surface of the photoconductor drum 1.
The toner image on the peripheral surface of the photoconductor drum 1 is primarily transferred in a primary transfer region onto a peripheral surface of the transfer roller 2 under a bias voltage applied through the transfer roller 2 and under a nip pressure between the photoconductor drum 1 and the transfer roller 2. The toner image so primarily transferred is secondarily transferred in a secondary transfer region onto a medium 7 passing between the transfer roller 2 and the backup roller 3. In the apparatus shown, there are also provided a photoconductor cleaner 8a for removing a toner residual on the photoconductor drum 1, a transfer roller cleaner 8b for removing a toner residual on the transfer roller 2, a static eliminator 9a and a carrier liquid supply unit 9b. 
In the electrophotographic apparatus of this sort, if the toner image formed on the peripheral surface of the photoconductor drum 1 is positively charged, the transfer roller 2 and the backup roller 3 are to have a voltage applied thereto that is of a polarity opposite to the charging polarity of the toner on the peripheral surface of the photoconductor drum 1, i.e., a negative voltage on the backup roller 3 made more negative than that on the transfer roller 2. Thus on the medium passing between the rollers 2 and 3 there is transferred the toner image on the peripheral surface of the transfer roller 2 under a nip pressure between the two rollers and under a potential difference between the two rollers.
Thus in such an electrophotographic apparatus, if the medium 7 used is high in insulating property tending to obstruct the potential difference between the transfer roller 2 and the backup roller 3, e.g., if it has a large thickness, the transfer between the transfer roller 2 and the backup roller 3 tends to be not made by the potential difference between the two rollers, there being no transfer thereby, but only by the nip pressure between them, there being a transfer only thereby, giving rise to the problem that the transfer is not done well. While when the medium is thick it is conceivable to achieve a potential difference enough to transfer the toner image by increasing an absolute value of the voltage applied to the backup roller 3, in the state that the medium 7 is not passed between the transfer roller 2 and the backup roller 3 the increased voltage in absolute value tends to bring about a discharge to the side of the transfer roller 2, leading to the problem that rubber on the transfer roller 2 may be destructed.
Made in view of the foregoing, the present invention has for its object to provide an electrophotographic apparatus that can effect favorable transfer of a toner image onto a medium under a nip pressure between a transfer roller and a backup roller and a potential difference between the transfer and backup rollers even if the medium used is high in insulating property such as by being thick in thickness.